1. Field of the Invention
The present invention relates to light emitting devices and, particularly, to LEDs and a method of manufacturing LEDs designed to having a uniform potential across the area of the light-emitting junction.
2. Discussion of the Background
There are a number of methods for the fabrication of discrete LEDs. Although the fabrication of LED arrays are constrained by issues related to the fabrication of discrete LEDs, there are additional constraints which must be considered when optimizing the fabrication of LED arrays.
For example, discrete LED arrays are typically fabricated on a conducting substrate since isolation between devices is not a concern because the devices will be separated as part of the packaging process. Conversely, LED arrays may require isolation between elements which is complicated by the use of a conducting substrate compared to the use of a semi-insulating substrate. Although a conducting substrate can be used, the fabrication is complicated and typically requires additional isolation and/or planarization to ensure that proper electrode isolation is achieved.
In addition, discrete LED fabrication typically uses a top contact near the center of the LED and a bottom contact around part or all of the periphery of the device. This approach minimizes issues associated with spreading resistance of the bottom contact due to the thickness of the conducting substrate but spreading resistance associated with the top contact can result in a non-uniform voltage at the pn junction resulting in LED brightness nonuniformity.
Since the pn junction is forward biased during operation, and the pn junction has an exponential current characteristic in the forward bias mode, a small difference in junction voltage significantly changes the operating current. According, the light emitted by the pn junction is nonuniform. Typically, this is manifested by a greater brightness nearer the top contact.
Solutions to the spreading resistance problem of the top contact have included the use of transparent contacts and additional process steps to minimize the spreading resistance between the top ohmic contact to the device and the pn junction. Thick epitaxial layers and/or semi-transparent contacts have been used. These solutions are not ideal since 1) transparent contacts typically exhibit some spreading resistance and these transparent contacts are not as transparent as no contact at all and 2) the growth of thick epilayers adds additional cost and complexity to the process and results in a thick device. For example, currently a high brightness LED is being manufactured having a very thick layer of GaAsP on top of the LED, but the device (epitaxial layers) is about 10 xcexcm thick.
It is an object of the present invention to provide an LED and a method of manufacturing an LED where the spreading resistance in the upper and lower layers of the LED structure is matched to obtain a uniform voltage across the pn junction of the LED structure.
Another object of the present invention is to minimize the spreading resistance in the upper and lower layers of a LED structure to simplify the matching of spreading resistance and achievement of a uniform voltage across the pn junction of the LED structure.
Yet another object of the present invention is to allow light emitting devices to be fabricated on a semi-insulating substrate to simply the fabrication process.
A further object of the present invention is to reduce the absorption (bandgap and free carrier) in the top layer of an LED structure while matching the spreading resistance in the upper and lower layers of the LED structure.
A still further object of the present invention is to minimize the thickness of an LED structure while matching the spreading resistance of the upper and lower layers of a LED structure to simplify planarization in the fabrication of LEDs and LED arrays.
These and other objects of the present invention are achieved by a light emitting device where contacts are made to the light emitting junction. The spreading resistances of the contacts are substantially equal, resulting in a substantially uniform voltage across the light emitting junction and substantially uniform light emission by the junction. The spreading resistance can be substantially matched by controlling the composition, doping and/or layer thicknesses of the contact layers.
The junction can be a pn junction having a p-type active layer and an n-type clad layer. The contact to the active layer may include a clad/grad layer or layers and a p contact layer, while the contact to the n clad layer may include an n-type grade layer and an n-type contact layer.
The layers through which light is emitted should be made transparent. This may be accomplished by making these layers thin, controlling their composition, or selectively removing portions of these layers in the light emitting area. When the layers are selectively removed, the spreading resistance matching can be accomplished with the remaining layers and/or layer portions.
The above and other objects of the invention may also be achieved with a method of fabricating light emitting devices where contacts to a light emitting element are formed have substantially the same spreading resistance. By forming the spreading resistances of the contacts to be substantially equal, a substantially uniform voltage across the light emitting element is produced, resulting in substantially uniform light emission by the element. The spreading resistance can be substantially matched by controlling the composition, doping and/or layer thicknesses of the contact layers.